The present invention relates in general to prefabricated reinforced concrete elements, whether precompressed or not, for the construction of works sunk beneath ground level such as artificial tunnels, underpasses, underground carparks and the like.
The majority of such works can be constructed only after open air excavations, and after having gone down to the level of the foundation or the floor of the work itself. Once the construction has been completed this must be covered over with earth in such a way as to form the embankment over the roof or the deck slab over the work and to sustain the supporting uprights at their flanks.
In many cases however the excavation cannot be performed in the conventional open air manner down to the base of the structure due to the lack of space for the banks at the sides thereof or else because the excavation involves risks for the stability of nearby works; in other cases the conventional excavation, with lateral banks is not economically convenient due to the large volume of ground to be excavated.
In order to avoid the formation of banks in the above-mentioned cases the sides of the excavation can be supported by means of containment structures known per se such as bentonitic diaphragms, piling, tieback pile walls, sheet piling or the like. Some of such containment structures, such as sheet piling or tieback pile walls, are used in a temporary manner only to permit the excavation to proceed, while the sides of the work to be produced, for example the walls of a tunnel or the shoulders of an underpass, are usually constructed working close to the temporary containment structure.
If it is intended to form the sides of a tunnel, the shoulder of an underpass or the wall of an underground carpark in situ, these can be constituted by bentonite diaphragms or by piling with large diameter piles. In these cases the prefabricated structure is constituted in practice only by the deck and by possible intermediate floors. The deck between two bentonite diaphragms or between two series of large diameter piles is in general formed by prefabricated rectilinear beams of precompressed reinforced concrete laid on a beam made in situ on each piling header or diaphragm header. The prefabricated deck beams are then integrated in situ by means of concrete castings which complete the cover between one beam and the next and which makes it possible to considerably improve the robustness of the individual prefabricated beams.
This arrangement, currently very widely used, has however several disadvantages. In the first place the prefabricated deck beams are not normally assigned the task of transmitting from one piling header to the next the horizontal force necessary to resist the thrust of the ground which acts on each piling so that the piling must be dimensioned in such a way as to resist these thrusts themselves.
Moreover, if the deck beams should be rigidly fixed between the two piling headers, the expansion of these beams due to thermal variations would impose on the piling headers themselves deformations which could involve excessive stresses in the pile because of the limited deformability of the ground in contact with the pilings. To overcome this problem the piling headers are fixed by anchoring them to the ground via anchors, or tie rods of precompressed reinforced concrete and often between the deck beams and the piling headers there is introduced an expansion joint which allows the effects of thermal expansion of the deck beams to be absorbed. This arrangement however weakens the hermetic seal of the work in correspondence with the roof and the deck of a road possibly overlying the work.
In the second place, the deck beams are frequently of large dimensions, which causes significant problems for their transport, because they must be simply supported both so as not to form a rigid tie with the piling headers and for constructional convenience. Because of this type of fixing between these beams and the piling headers it is not possible to take advantage of the benefits due to the embedded bending moment at the supports of the beams so that these must be dimensioned also taking this disadvantage into account.